Navigation and positioning systems are in common use, for example using one of the available Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS). In such systems, a user device receives signals from multiple satellite vehicles, each of which has a predetermined trajectory. As a result, the user device is able to make measurements on the received signals, and is able to use these to provide estimates of the user's position and velocity.
One well-known problem with such systems is that signals from the satellite vehicles can arrive at the user device after being reflected off, or refracted by, obstacles that are in the line-of-sight path between the satellite vehicle and the user device or are close to the line-of-sight path. These obstacles can also have the effect that the direct line-of-sight signal is attenuated significantly, possibly to the extent that the user device is practically unable to receive it.
The effect of this problem can be that the user device is unable to make good estimates of the user's position and velocity.
US-2008/0238772-A describes a positioning system, for use in an urban environment in which there are many potential obstacles such as tall buildings. In the described system, the positions of the buildings are stored in the system. The position of the device is then determined by using signals from an inertial navigation system, and from a satellite positioning system, with the satellite positioning system predicting the characteristic of a multipath signal reflected off one of the known obstacles. This requires detailed knowledge of the locations of the potential obstacles.